1. Field of the Invention
The present invention relates to bonding a transparent plastic article to an opaque plastic article. More specifically, the present invention is directed to a method of bonding a transparent article to an opaque article by directing an infrared beam through the transparent article to heat and soften the opaque article.
2. DESCRIPTION OF THE RELATED ART
Most automotive lens assemblies are manufactured using a clear lens and an opaque reflector. The reflector may be metallized to increase the reflectivity and the performance of the assembly. The lens and reflector are joined by placing an adhesive between mating surfaces. The adhesive generally fills a U-shaped groove in one of the components. A projecting tongue is placed within the groove. The adhesive bonding process described is sufficient to provide a leak-free attachment of the lens to the reflector. There are several disadvantages to using an adhesive between the transparent and opaque members. The adhesive is costly and difficult to apply. It must be carefully dispensed so as not produce areas that are too thick and became squeezed into the reflector or too thin and provide insufficient material to join the components. Also, the adhesives require a period of time to cure. Generally, the lens and reflector must be urged together while the adhesive cures thus increasing the manufacturing cycle time. It is generally desirable to provide a method of attaching the transparent lens to the reflector without the need for adhesives or sealant.
Another method of securing a lens to a reflector is described in commonly assigned U.S. patent application Ser. No. 09/174,611, filed Oct. 19, 1998, titled "METHOD AND APPARATUS FOR MANUFACTURING A TWO-PIECE PLASTIC ASSEMBLY" and incorporated herein by reference. This patent application teaches joining two plastic articles by injecting a plastic between the two components. The injected plastic fuses to and joins the components in a mold. This process utilizes the injected plastic to join and seal the lens and reflector. The present invention is designed to join and seal the lens and reflector without the additional plastic sealing material.
Several processes exist to join plastic articles without adhesives or plastic sealers. Among them are methods such as hot air bonding, sonic welding, vibration welding, mechanical fasteners and infrared radiant heat.
One method in particular, infrared (IR) radiant heat, has certain advantages over the other types. Generally, there is a desire to form a strong bond between the two parts quickly and at a minimal cost. Additionally, it is preferable that the bonding process avoids touching the surfaces to be bonded themselves where they are to be bonded in order to assure uniform bonds from part to part and to reduce the cost of cleaning the bonding apparatus. Further, in many instances, it is preferable that there be no marring or distortion of the surface on the opposite side from that which is bonded, herein referred to as the viewing surface.
Infrared bonding in general can overcome many of these concerns. Infrared energy can be finitely pinpointed with a focal point or a mask to the exact area to be bonded in order to avoid overheating the plastic in adjacent areas that might cause unwanted distortion in the part. Infrared can heat the bonding surface to a high temperature; thus assuring a strong bond is formed between the two parts. The bonding surface can be heated very quickly with infrared and the timing and amount of heat application can be precisely controlled. Further, with infrared heating, there need be no contact between the heat source and the bonding surfaces of the parts, in order to minimize cleaning requirements for the tooling.
One method of bonding two opaque articles is described in commonly assigned U.S. Pat. No. 5,522,954 ('954), issued Jun. 4, 1996, and incorporated herein by reference. This patent teaches a method of heating two opaque surfaces using an infrared heat source that produces low and high heating. The use of varied heating precluded damaging the articles by the heat of the infrared beam. This method has been successfully used for bonding opaque parts that are directly exposed to the infrared beam. The '954 patent does not describe the present process of passing an IR beam through a material that is transparent to the heating effects of the IR beam.
Other patents also describe various methods of bonding articles using infrared heating. U.S. Pat. No. 5,151,149 to Swartz discloses a method and apparatus for heating plastic for bonding using infrared, which attempts to heat the plastic fairly quickly up to its bonding temperature while avoiding burning the viewing surfaces. It discloses using infrared focused heat sources that move rapidly in repeated patterns to heat the plastic at a given point a little more on each pass.
Accordingly, it is desirable to infrared bond plastic parts that have been assembled without the need for adhesives or sealers. It is also desirable to join plastic parts while having access to only one surface of the assembly.